sf2: add NOTES about using yosys for smartfusion2 and igloo2
sf2: add NOTES about using yosys for smartfusion2 and igloo2

YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys